A valve operating device provided with adjustment mechanisms for adjusting a valve open timing of an intake valve and valve close timing of an exhaust valve at the same time as adjusting a maximum lift amount of the intake valve and the exhaust valve is known from the disclosure of JP 2000-3721 A. The valve operating device is provided with an eccentric cam fixed to a drive shaft rotating in response to rotation of a crankshaft, an annular link rotatably engaged with the outer periphery of the eccentric cam, a rocker lever which is rotatably engaged with the outer periphery of a control cam that is fixed in an eccentric manner to a control shaft arranged substantially parallel to the drive shaft and which is pivoted about the annular link at one end thereof, and a rocking cam rotatably engaged with the drive shaft and connected to the other end of the rocker lever via a link.
The rocking cam for opening and closing operations of the intake valve and the exhaust valve oscillates to vary a maximum lift amount and an operating angle of each of the intake valve and the exhaust valve when variation of a distance between the rocking center of the rocker lever and the rotational center of the drive shaft occurs when the control shaft rotates in accordance with engine driving conditions. The control shaft is rotationally controlled in such a manner that as the maximum lift amounts of the intake valve and the exhaust valve become smaller, the maximum lift timing is moved towards being retarded at the intake valve, and moved towards being advanced at the exhaust valve. As a result, the valve open timing of the intake valve is retarded by an amount that is larger than the amount of advance of the valve close timing of the intake valve, while the valve close timing of the exhaust valve is advanced by an amount that is larger than the amount of retardation of the valve open timing of the exhaust valve, and it is thus possible to improve fuel consumption rate and purify the exhaust gas by using combustion gas retained in the combustion chamber.
On the other hand, SAE TECHNICAL PAPER SERIES, 2000-01-1221, (Mar. 6–9, 2000), “Design and Development of a Mechanical Variable Valve Actuation System” by Ronald J. Pierik and James F. Burkhard discloses a valve operating device for an internal combustion engine provided with an adjustment mechanism for advancing valve open timing of the intake valve as the maximum lift amount of the intake valve becomes smaller.
Here, the intake valve is opened and closed by a valve operating device provided with an adjustment mechanism, and the exhaust valve is opened and closed by the valve operating device that is not provided with a characteristic adjustment mechanism. The adjustment mechanism has an input cam provided on a cam shaft synchronized with a crankshaft, an output cam supported on a cam shaft, a frame pivoted on the cam shaft, a link with one end pivoted on the output cam so as to be capable of swinging, a rocker lever having a roller coming into contact with the input cam, and having one end pivoted on the frame and the other end pivoted on the link so as to be capable of swinging, and a control shaft for causing the frame to oscillate. As the maximum lift amount of the intake vales become smaller, the valve open timing is kept substantially the same, while the valve close timing is advanced.
With the valve operating device disclosed in JP 2000-3721 A, a movement angle at the maximum lift timing is determined by an angle through which the swinging center of the rocker lever rotates with respect to the center of rotation of the drive shaft, when the control cam is rotated by the control shaft. However, since the rocker lever is rotatably supported by a control cam fixed to the control shaft positioned away from the drive shaft supporting the rocking cam, the rotational angle of the oscillating center of the rocker lever around the rotational axis of the drive shaft is dependent on the amount of eccentricity of the rocker lever and is limited to a small value. This means that it is difficult to cause a large amount of combustion gas to be retained in the combustion chamber by carrying out a large amount of retard of the intake valve open timing and a large amount of advance of the exhaust valve close timing, thus increasing the duration from the closing of the exhaust valve to the opening of the intake valve during the period from the exhaust stroke and to the intake stroke (referred to hereinafter as “negative valve overlaping duration”).
On the other hand, the valve operating device disclosed in “Design and Development of a Mechanical Variable Valve Actuation System” mentioned hereinabove is not provided with a mechanism for adjusting the valve close timing in the exhaust valve operating device, and it is difficult to retain a sufficient combustion gas in the combustion chamber because the intake valve is opened and closed without substantial change in the valve open timing of the intake valve even if there is a change in the maximum lift amount so that the valve overlapping duration hardly changes.
The present invention has been made in view of the above described situation, and the main object of the invention is to provide a valve operating device capable of increasing the negative valve overlapping duration when the maximum lift amounts of the intake valve and the exhaust valve are small. The present invention further contemplates making compact a characteristic adjustment mechanism for adjusting each of the operating characteristics of the intake valve and the exhaust valve, and also further simplifying the structure thereof and causing a decompression operation by the characteristic adjustment mechanisms.